Hi I keep demansoni with Crabro, no problems at present.
as for catfish what about Phylonemus typus, they look different, and seem fairly social and non agressive
Try Amquel by kordon, it is not widley available so you may have to search around
Ammonia and no nitrite is a bad thing as it probably means the filter is not working at all (biologically).
KH or GH
Carbonate containing compounds such as Sodium bicarbonate will raise KH
To raise GH use calcium carbonate (sparingly soluble), combined with magnesium sulphate, and or calcium sulphate.